1. Field of the Invention
This invention relates to a reamer for precision finishing a rough drilled bore in a metal workpiece.
2. Description of the Prior Art
There has been known a reamer comprising a shank portion, a reamer head adapted to be removably fixed to the shank portion, a cutting blade provided on the reamer head to project axially forwardly from the front end of the reamer head, a bearing guide pad provided on the reamer head circumferentially spaced from the cutting blade by 90.degree., and a sizing guide pad provided on the reamer head diametrically opposed to the cutting blade as disclosed in U.S. Pat. Nos. 3,880,545; 4,076,445 and 4,125,342 and Japanese Unexamined Utility Model Publication No. 61(1986)-184611, for example. The bearing guide pad bears the main cutting force of the cutting blade and the sizing guide pad together with the cutting blade determines the diameter of the bore to be drilled by the reamer, that is, the diameter of the bore depends on the distance between the outer edge of the sizing guide pad and the cutting edge of the cutting blade Further, the guide pads serve to burnish the cylindrical wall of the drilled bore.
Since the reamer is for precision finishing a rough drilled bore, the roundness and straightness of the bore finished by the reamer are required to be high and the diameter of the bore must be within a narrow limit. Conventionally, it has been considered that such requirements can be met by elongating the guide pads in the axial direction of the reamer. However, it has been found that, though the straightness and the roundness of the reamed bore can be improved to some extent by elongating the guide pads, it is limited and the requirements can be better met by making the guide pads shorter than the axial length of the cutting blade rather than elongating the guide pads. It may be considered that this is because, when the axial lengths or the guide pads are large, the frictional resistance between the guide pads and the wall of the bore increases as the reaming progresses to exert a strong radial constraining force on the reamer which acts as an adverse component on the reamer overcoming the thrust constraining force and the cutting force exerted by the cutting edge, whereby the straightness and the roundness of the reamed bore are deteriorated as well as the precision of the diameter of the reamed bore.
In other words, the tolerance of the deviation in the inclination of the reamer can be enlarged by making the axial length of the guide pads and circle land of the cutting blade small to reduce the constraining force on the reamer of the inner wall of the bore which is being reamed and enable the reaming of the bore having high roundness and straightness.
Further, the roundness of the reamed bore can be improved by making the width of the sizing guide pad narrower than that of the bearing guide pad.